The families of Eamon Fox and Gary Convie, two Catholic workmen murdered by the UVF 23 years ago, will be hoping that the Director of Public Prosecutions is correct to be optimistic that there is a reasonable prospect of conviction.

The families of Eamon Fox and Gary Convie, two Catholic workmen murdered by the UVF 23 years ago, will be hoping that the Director of Public Prosecutions is correct to be optimistic that there is a reasonable prospect of conviction.

While it would be improper for us to comment on the potential case against the suspect, it is public knowledge that part of the evidence will be given by assisting offender Gary Haggarty, a former UVF commander.

The use of assisting offenders - or supergrasses as they used to be known - is a tactic which has had a chequered past in Northern Ireland.

Its use during the darkest days of the Troubles in the 1980s served a dual purpose.

It led to hundreds of suspected republican and loyalist terrorists being arrested and held on remand for considerable periods, giving the hard-pressed security forces valuable breathing space and the ability to regroup during times of constant violence. And it did result in some successful prosecutions.

However, it was a tactic which was eventually discredited and largely abandoned after the judiciary complained they were being used for political purposes. But it has been resurrected in recent times as police sought to bring to justice a UVF gang based in the Mount Vernon estate on the outskirts of north Belfast suspected of involvement in a large number of killings - mostly sectarian - and also said to be deeply penetrated by RUC Special Branch informers.

It has met with little success. In 2012 two brothers who became assisting offenders were branded liars by the judge and 12 of the 13 men in the dock walked free.

Haggarty, who agreed to become an assisting offender, pleaded guilty to a series of crimes including murder and has already spent three years in custody.

He was interviewed by detectives more than 1,000 times producing huge volumes of evidence but the lack of collaboration led to a decision earlier this year not to prosecute 13 men. Another murder case against two other men was dropped later.

Bringing sophisticated terrorists to justice is a difficult task as the legacy of the Troubles proves with almost 3,000 unsolved murders, and the police and legal authorities are entitled to use every legitimate method to bring some comfort or closure to the bereaved. But they must be careful not to add to families' pain by raising hope and producing failure.